1. Field of the Invention
The invention relates to a video system, and more particularly, to a method and apparatus for remote digital slow shutter video processing of video signals in a video system. This method is particularly, though not exclusively, suited for use in video surveillance systems.
2. Description of the Prior Art
In video surveillance situations, it is oftentimes desirable to monitor a number of remote locations, such as entrances and exits of a building or stations along a production line, from a centralized monitoring location. For these situations, separate video cameras are stationed at each respective location to produce a view of the monitored location. If the view on each camera changes slowly, it is possible to use a single monitor to display on a time-shared basis the images produced by the cameras.
A conventional television system transmits a video signal containing a series of vertical synchronization (synch) pulses which occur approximately every 1/60th of a second ( 1/50th of a second in Europe). The vertical synch pulses provide timing information for the vertical sweep or deflection signal used to scan a cathode ray tube (CRT) to reconstruct the complete video image. If a vertical synch pulse is missed, the vertical sweep circuit responsive to the vertical synch pulses will come “out-of-lock” with the vertical synch pulses. An amount of time lasting through many vertical synch pulse intervals is often required for the vertical sweep circuit to re-lock onto the incoming vertical synch pulses. In addition, a conventional alternating current (AC) coupled sweep amplifier that drives the CRT is upset by the non-repetitive sweep input and hence rings and bounces for many vertical fields. During this transient, a blank bar is produced across the display of the television receiver or monitor, and the location of the image being displayed on the receiver or monitor bounces and rolls across the screen.
A camera uses an image sensor to acquire an image. The image sensor may be a tube-type sensor or solid-state sensor. The image sensors are typically designed to operate in daylight. In low light conditions, the image sensor may not receive sufficient light to produce a visually acceptable image in 1/60th of a second. To compensate, the shutter speed may be slowed to increase the exposure time of the image sensor. However, reducing the shutter speed results in the transmission of a new image at intervals exceeding 1/60th of a second and will result in a non-standard video format and synch pulses. In addition, the displayed image may flicker.
It has been demonstrated that a digital refresh memory can be built into a camera to provide the display refresh function to improve the video system's performance under low light conditions. In order to provide a sufficient amount of light to the image sensor, the shutter speed of the camera is reduced. The camera includes an analog-to-digital converter to digitize the signal from the image sensor, which is then stored in the refresh memory. The refresh memory stores picture element (pixel) data representative of the input signal. Typically, the refresh memory is a dual-port random-access memory (RAM) that, for example, is of sufficient size to store the pixels of a complete television (TV) frame, that is, two interleaved fields. The refresh memory is updated at the shutter speed of the camera, while the pixel data is read from the refresh memory every 1/60th of a second. The image data read from the refresh memory is converted to analog form, and transmitted with a vertical synch pulse as an analog video signal. In this way, cameras provide video images of sufficient quality under low light conditions, and continue to supply standard rate ( 1/60th of a second) vertical synch pulses.
Providing a refresh memory in every camera of a video system is expensive. With the introduction of advanced digital processing techniques, the video pictures generated by the cameras are processed digitally in order to store or resynchronize the image. Consequently, digital memories having large storage capacity and high input and output data rates are required. However, large memories with fast data rates are generally costly. For example, a typical video surveillance system may have 500 cameras and a much smaller number of displays. Including a refresh memory in each of the 500 cameras incurs a significant cost.
Video surveillance systems, such as closed-circuit television (CCTV) systems generally include components that are designed to provide a specific complete self-contained function, such as cameras and monitors. However, cost and performance improvements can be achieved by placing some camera and monitor functions in a central location.
Therefore, there is a need for a method and apparatus to provide an effective slow shutter capability in a video system at a reduced cost. The method and apparatus should also operate with existing video components.